Engine brakes or retarders are used to assist and supplement wheel brakes in slowing heavy vehicles, such as tractor-trailers. Engine brakes are desirable because they help alleviate wheel brake overheating. As vehicle design and technology have advanced, the hauling capacity of tractor-trailers has increased, while at the same time rolling resistance and wind resistance have decreased. Thus, there is a need for advanced engine braking systems in today's heavy vehicles.
Problems with existing engine braking systems include high noise levels and a lack of smooth operation at some braking levels resulting from the use of less than all of the engine cylinders in a compression braking scheme. Also, existing systems are not readily adaptable to differing road and vehicle conditions. Still further, existing systems are complex and expensive.
Known engine compression brakes convert an internal combustion engine from a power generating unit into a power consuming air compressor.
U.S. Pat. No. 3,220,392 issued to Cummins on Nov. 30, 1965, discloses an engine braking system in which an exhaust valve located in a cylinder is opened when the piston in the cylinder nears the top dead center (TDC) position on the compression stroke. An actuator includes a master piston, driven by a cam and pushrod, which in turn drives a slave piston to open the exhaust valve during engine braking. The braking that can be accomplished by the Cummins device is limited because the timing and duration of the opening of the exhaust valve is dictated by the geometry of the cam which drives the master piston and hence these parameters cannot be independently controlled.
U.S. Pat. No. 5,012,778 issued to Pitzi on May 7, 1991, discloses an engine braking system which includes a solenoid actuated servo valve hydraulically linked to an exhaust valve actuator. The exhaust valve actuator comprises a piston which, when subjected to sufficient hydraulic pressure, is driven into contact with a contact plate attached to an exhaust valve stem, thereby opening the exhaust valve.
U.S. Pat. No. 4,572,114 issued to Sickler on Feb. 25, 1986, discloses an electronically controlled engine braking system. A pushtube of the engine reciprocates a rocker arm and a master piston so that pressurized fluid is delivered and stored in a high pressure accumulator. For each engine cylinder, a three-way solenoid valve is operable by an electronic controller to selectively couple the accumulator to a slave bore having a slave piston disposed therein. The slave piston is responsive to the admittance of the pressurized fluid from the accumulator into the slave bore to move an exhaust valve crosshead and thereby open a pair of exhaust valves.
Braking systems have been developed that control the lash take-up between an actuator and an exhaust valve.
Actuators in engine braking systems require a lash, i.e., a minimum cold clearance, between an actuator and the exhaust valve to prevent the exhaust valve from opening prematurely when the exhaust valve expands due to engine heat. The lash, however, affects the timing of opening and closing the exhaust valve. To overcome this problem, prior braking systems have employed methods that keep the valve-actuating mechanism engaged with the exhaust valve, thereby eliminating the lash.
For example, U.S. Pat. No. 4,898,128 issued to Meneely on Feb. 6, 1990, discloses an anti-lash adapter which includes a slave piston adapted to contact an exhaust valve crosshead. The slave piston is biased by springs disposed on opposite sides of the slave piston, and is further disposed in fluid communication with a master piston. The lash between the slave piston and the crosshead is taken up by the net forces acting on the slave piston before the master piston is displaced. Thereafter, displacement of the master piston causes the slave piston to force exhaust vales open via the crosshead.